In recent years, IETF (Internet Engineering Task Force) has been considering an IP mobility scheme for seamless movement capable of performing handover between a plurality of different wireless communication networks, such as a cellular phone network, a wireless LAN and the likes, so as to achieve a ubiquitous environment. As a specific protocol of the IP mobility scheme, there are Mobile IPv4 and Mobile IPv6 (which are abbreviated as Mobile IP, hereinafter) for supporting movement of an individual communication terminal, and NEMO (Network Mobility) for supporting mobility of a network as a unit.
Incidentally, when an application (hereinafter, abbreviated as APP appropriately) such as VoIP having a real-time property is executed via the wireless communication network, an allowable bandwidth of a wireless communication path changes depending on a propagation environment such as fading, and arrival intervals of packets received by the communication terminal are changed in accordance with a change of the allowable bandwidth.
For this reason, it is generally performed to provide the communication terminal with a jitter buffer so as to first store received packets in the jitter buffer and then read out the packets from the jitter buffer and reproduce the packets at intervals based on the application. Thereby, it absorbs deviation in the packets, that is, displacement of reproduction intervals of packets caused by displacement (jitter) of the arrival intervals of the packets, so as to prevent deterioration of reproduction quality such as reproduced sound quality and the likes. Moreover, when there is no packet in the jitter buffer since the jitter is large, and therefore silence occurs, or when too many packets are received in a short period to be stored in the jitter buffer, the communication terminal changes a reproduction speed, discards received packets, or changes a size of the jitter buffer (delay time).
On the other hand, a downlink absolute delay time of a packet received by the communication terminal, that is, a time required for a packet transmitted from a counterpart communication terminal to be received via the wireless communication network differs in accordance with wireless communication networks. Thus, when the communication terminal is a wireless communication apparatus which is moving and performs handover to a different wireless communication network, overtaking of received packets may occur when the downlink absolute delay time of a handover destination is shorter than that of a handover source, for example.
FIG. 8 is a diagram showing flows of the packets in such a case, showing transmission timings of the packets from the counterpart communication terminal (CN: Correspondent Node), transfer timings of the packets by a home agent (HA) to transfer the packets from the CN to the wireless communication apparatus (MN: Mobile Node), and reception timings of the packets by the jitter buffer of the MN. It is to be noted that it is assumed here that there is no deviation in the received packets (displacement of arrival intervals) at both of the wireless communication network A of the handover source and the wireless communication network B of the handover destination.
As a handover process by the MN from the wireless communication network A to the wireless communication network B, the MN transmits Registration Request (Binding Update for NEMO) to the HA via the wireless communication network B of the handover destination and registers a care-of address of the handover destination to the HA. Then, the MN receives Registration Reply (Binding Acknowledge for NEMO), which is handover completion information transmitted from the HA in response, cuts the connection with the wireless communication network A of the handover source, and then transmits/receives the packets via the wireless communication network B of the handover destination.
In this case, when the downlink absolute delay time TddnB of the wireless communication network B of the handover destination is shorter than the downlink absolute delay time TddnA of the wireless communication network A of the handover source by a predetermined value or more, it causes overtaking of the received packets for a period TAB (TddnA-TddnB) from a time of receiving the handover completion information. Hence, during this period TAB the number of packets received by the jitter buffer in a unit time becomes very high, as shown in FIG. 9(a).
In such a case, when the reproduction speed (read-out intervals) of the packets from the jitter buffer is a certain speed in accordance with the APP as shown in FIG. 9(b), the number of packets in the jitter buffer rapidly increases from completing handover to the wireless communication network B, as shown in FIG. 9(c). Thereby, the received packets exceeding a buffer size because of overtaking are discarded without being reproduced, and thus the number of packets in the jitter buffer settles down to a certain number of packets corresponding to the buffer size.
Discard of the packets as such causes deterioration of reproduced voice. When the buffer size of the jitter buffer is sufficiently large, the packets are not discarded but the packets from the handover source and the packets from the handover destination can be reproduced at scheduled timings. In such a case, however, the packets from the handover destination received overtaking the packets from the handover source stay in the jitter buffer longer than the packets from the handover source. As a result, delay times become excessively long, preventing achievement of a real-time property corresponding to the delay time of each of the wireless communication networks.
As a method capable of improving such a defect at handover, for example, there is suggested a control method of the jitter buffer to monitor reception conditions of packets and controls a speed to read out the packets from the jitter buffer based on a result of monitoring (for example, Patent Document 1).
FIG. 10 shows diagrams illustrating an operative condition of the jitter buffer by the control method of the jitter buffer disclosed in Patent Document 1. FIGS. 10(a) to (c), in the same manner as FIGS. 9(a) to (c), show the number of received packets by the jitter buffer in a unit time, the reproduction speed and the number of packets in the jitter buffer, respectively.
In FIG. 10, when the packets are received at shorter intervals than previous intervals such as a case where handover is performed from the wireless communication network A with the downlink absolute delay time TddnA to the wireless communication network B with the downlink absolute delay time TddnB shorter than the TddnA, the reproduction speed of the packets is set high in accordance with shortened reception intervals. Thereafter, the reproduction speed is controlled so as to gradually slow down to the normal reproduction speed in accordance with the number of packets in the jitter buffer.    Patent Document 1: Japanese Patent Laid-Open No. 2006-238445